The present invention relates to a method for determining in a gas mixture the concentration of at least one gas component, which gas mixture contains or may contain component materials which cause collision dilation or boardening of the absorption lines constituting the absorption spectral band of the gas component, said method comprising: (a) providing a radiation source for emitting radiation through a gas mixture to be analyzed over a wavelength range, in which is included the absorption spectral band used in the concentration measuring of said gas component, providing the path of radiation progressing through the gas mixture with an optical bandpass filter, having a transmission band which positions itself in line with said spectral band and using a detector for detecting the radiation that has passed through the gas mixture and the optical bandpass filter; (b) carrying a first signal produced in the detector by this intensity to a calculating unit for obtaining a first result based on non-dispersive radiation absorption measuring. The invention relates also to a measuring device for implementing this method.
When measuring a gas concentration with infrared technique, the most common practice is to use a non-dispersive method, which means that the absorption signal is measured over a narrow wavelength range, i.e. through an optical bandpass filter provided with a narrow transmission band. Thus, the measured signal representing the radiation that has passed through a sample and an optical filter will be an integrated value of transmissions occurring at various wavelengths of the band. The absorption spectrum of a gas in molecular state normally consists of absorption bands produced by molecular vibrations and of a fine structure, i.e. absorption lines, resulting from rotational transitions inside the molecule. Thus, when measured at a sufficient resolution, the absorption spectrum of a gas consists of a large number of very narrow absorption lines. For example, carbon dioxide has a vibration absorption range, whose spectral band has a mean wavelength of 4260 nm. A more accurate analysis confirms that the band consists of more than 80 narrow absorption lines produced by rotation. These lines have a half-value width and intensity which are dependent on several factors, such as temperature, self-absorption resulting from a long measuring path, and collisions by other molecules included in a gas mixture. The first two are generally quite easy to account for in the compensation of a measuring signal by measuring the temperature and the linearization effects on a gas in question caused by measuring geometry. On the other hand, the alteration resulting from collisions by other gas components, i.e. a mixture gas effect, which sometimes can be significant, must be taken very carefully into consideration for the minimization of concentration errors.
The publication JOURNAL OF APPLIED PHYSIOLOGY, Vol. 25 No. 3, 1968, pp. 333-335: Ammann, Galvin--"Problems associated with the determination of carbon dioxide by infrared absorption" discloses how the measured concentration value of carbon dioxide changes in different gas mixtures. The obtained result is typical when measuring is effected nondispersively with an optical filter having a narrow transmission band, said transmission band extending, as per normal, across several rotational lines. In the collisions of gas molecules, the energy spectrum of a rotational transition increases, resulting in the dilation or broadening of an absorption line. In measuring, it is detected as an increased absorption, even if the absorbances of rotational lines integrated across the measuring band were in fact invariable. This is due to the fact that the object of measuring is transmission instead of absorbance. The absorbance value, which is calculated on the basis of transmission and which is proportional to concentration, differs from a correct value just enough to overestimate the concentration. If the calibration of carbon dioxide is performed by using a nitrogen compound therefor, as in the cited publication, the concentration measured for certain other gas mixtures is too high and for others it is too low.
Especially polar gases, such as nitrous oxide, have a major effect on the line width. The magnitude of a concentration error is also influenced by the length of a measuring duct. With a short duct, i.e. with a short absorption length for a gas mixture to be measured, the demand for compensation is lesser whereas with a longer duct, wherein the absorption of a gas to be measured has decreased transmission substantially more, the demand for compensation can be considerable. When measuring e.g. carbon dioxide from the alveolar air of a patient, whereby a considerable portion of the gas mixture may consist of nitrous oxide (laughing gas), the calculated concentration value can be even 15% too high as a result of the above-explained error caused by transmission measuring. Therefore, for example, compensation of the amount of carbon dioxide measured in the alveolar air of a patient is conducted by measuring separately the content of laughing gas and by implementing on the basis thereof a mathematical correction of the carbon dioxide concentration, as described in the publication U.S. Pat. No. 4,423,739. Likewise, the effect of oxygen and the effect of anesthetic gases used during anesthesia are often compensated for although, in practice, the error caused thereby is slightly lesser. A problem with the method described in the cited publication is that all gases contributing to the collision dilation must be measured separately or concentrations of the gases must be otherwise known. This is inconvenient and unreliable, especially since the mixture includes varying amounts of hard-to-measure gases, such as nitrogen or in some cases helium or argon.
The publication GB 2,218,804 endeavours to provide a correction signal for the collision dilation by measuring thermal conductivity. It seems that the collision dilation produced by nitrous oxide in carbon dioxide is presented in the cited publication in a wrong direction. In reality, when the concentration of nitrous oxide rises, the measured absorption of carbon dioxide increases and this, in turn, increases the uncompensated concentration of carbon dioxide. In fact, there is no evidence to indicate that thermal conductivity in general would actually correlate in any way with collision dilation, although this seems to be the case in terms of the discussed gas mixture. Nitrous oxide has a low thermal conductivity, which must be proportioned to its major contribution in carbon dioxide in terms of promoting collision dilation. However, nitrogen and oxygen have both roughly the same thermal conductivity, although nitrogen contributes more than oxygen towards collision dilation. A result of this is that in alveolar air, for example, a measuring error of CO.sub.2 resulting, for example, from various concentrations of oxygen cannot be corrected. As an example, a measuring process has been applied to a mixture containing 5% of carbon dioxide in nitrogen, oxygen, and nitrous oxide by using a non-dispersive infrared measuring device. Relative to the tendency of nitrogen to dilate the absorption lines of carbon dioxide, it was confirmed that oxygen yielded 6% lower concentration values for carbon dioxide. On the other hand, nitrous oxide yielded carbon dioxide with values that were 7% too high. This can be contributed to the fact that oxygen has a thermal conductivity which is just 0.7% higher than that of nitrogen, while nitrous oxide has a thermal conductivity which is 37% lower than that of nitrogen. This leads to a conclusion that it is totally impossible to create a reliable correction method by the application of thermal conductivity. In addition, there are anomalies, such as hydrogen, which has a very high thermal conductivity, but still has a collision-dilation promoting effect on carbon dioxide which is even stronger than that of nitrous oxide.